This invention relates to lasers and more particularly to thermally driven lasers employing thermionic processes.
Lasing in a material depends on the formation of population inversions between specific atomic or molecular states. That is, laser activity, the production of coherent radiation, becomes possible when the number of atoms or molecules in excited states exceeds the number in lower states. Such a condition is known as a population inversion. An atom is said to be in an excited state when one or more of its electrons has been elevated to an energy level above the lowest state. Transitions of electrons from excited states to lower states result in the emission of photons of electromagnetic radiation. In a laser, the emission of a photon from a transition within a single atom stimulates transitions in other excited atoms so that the photons which they emit are in phase, thereby generating a laser's characteristic coherent radiation.
In order to create the population inversions, energy must be added to excite or "pump" the lasing material to a higher energy state. It is known to pump lasing materials with electrical, optical, chemical or nuclear energy sources. These methods of laser pumping, however, are both complex and energy inefficient. For example, the efficiency of a gas laser excited by radio frequency electromagnetic energy has an efficiency of approximately 0.1%.
It has therefore been of prime interest to develop a laser which may be pumped by thermal energy alone, generated, for example, from the combustion of fuels or from nuclear or solar energy. Such a laser would be particularly useful in situations where a suitable source of thermal energy is available. One such application is solar or nuclear powered optical inter-satellite communications networks. Other applications suited to thermally pumped lasers include the rapid switching of high electrical powers, radioactive isotope separation, ultra-sensitive pollution detection, accurate monitoring of industrial processes and the chemical analysis of trace elements. In addition, thermally pumped lasers would deliver substantially higher energy efficiencies than optically or electrically pumped lasers and would require no input other than thermal energy. The higher conversion efficiencies result from the fact that thermal energy is used directly rather than being converted first to electricity for electrical discharge excitation of a lasing medium, or further converted to light suited for optically exciting a lasing medium.
Although the desirability of thermally pumped lasers has been long recognized, their development has been frustrated because high translational temperatures, which are a measure of the degree of motion of the atoms or molecules in the lasing medium, tend to destroy the population inversions before lasing can occur. This is so because the energetic collisions among the constituents tend to bring the medium into thermal equilibrium. It is known from Boltzmann's law that thermal equilibrium at any temperature requires that a state with a lower energy be more densely populated than a state with a higher energy. Thus, thermal equilibrium is inconsistent with population inversions required for laser activity.
It is an object of the present invention therefore to provide a laser pumped by thermal energy alone.
It is another object to provide such a laser which operates both in a pulsed and in a d.c. mode.